The present invention relates to injection devices and methods of injecting medicinal products. More particularly, it relates to a piston stopper for injection devices, which serve to inject a medical or therapeutic product, and to product containers for such a medical or therapeutic products. Further, it relates to product containers and injection devices, each comprising a piston stopper in accordance with the invention. The application claims the priority of German patent application No. 102 26 643.3, filed on Jun. 14, 2002 with the German Patent and Trademark Office.
Piston stoppers of a generic type are known. For example, the piston stopper of DE 33 25 622 A1 is inserted into a syringe cylinder serving as a product container for the product to be injected, for example a liquid comprising a medical or therapeutic active agent. If the piston stopper is axially advanced into the product container, then the product to be injected is forced out through a delivery opening at the proximal end of the product container, where an injection needle is usually attached. The piston stopper comprises a stopper body holder which supports a stopper body which is in direct contact with the product stored in the product container and which seals off the product container. For the piston stopper to have a sealing fit in the product container, the stopper body is formed from an elastic material which is circumferentially supported against the inner circumference of the product container. The outer dimensions of the stopper body are slightly larger than the inner dimensions of the product container, such that the stopper body fits in the product container, sealed by elastic distortion.
The necessary elasticity of the stopper body causes a certain inertia in the response characteristics of the piston stopper. It does not respond immediately when a force is applied to axially advance the piston stopper. Until the static frictional force is exceeded, the applied forces predominantly result in a distortion of the stopper body, but not a delivery of the product to be injected. Thus, although a driven member of the injection device which advances the stopper body is axially advanced towards the product container, product is not delivered. When the static frictional force is exceeded, the elastic distortion is initially partially reduced since the sliding friction between the stopper body and the inner wall of the product container is typically smaller than the static friction. If the amount of product actually delivered from the product container over time and the actual adjusting path of the driven member of the injection device are plotted, peaks are identified which are difficult to control, in particular in micro-dosing. These peaks affect precision, in particular in micro-dosing systems in which the product to be injected is to be administered quasi-continuously in a multitude of relatively small doses over an extended period of time. The height of the peaks observed has an effect on the minimum achievable individual dose of an injection device or such a micro-dosing system.
The inertia in the response characteristics caused by the elasticity of the stopper body is also disadvantageous in the event of a blockage in areas downstream of the injection device, for example in blockages in the injection needle or in hoses arranged downstream, since it is possible to inadvertently deduce from an actually observed adjustment of the driven member of the injection device that a product has been delivered, although no product has actually been delivered due to the blockage, rather the stopper body has merely been distorted.